The invention relates to a magnetic recording device having a plurality of recording heads arranged in a matrix-type structure.
In digital magnetic tape recording systems for audio, video or data signals, helical-track recording and longitudinal-track recording processes are known. In longitudinal-track recording, multitrack recording is becoming increasingly significant. In addition to the classic formats S-DAT and DCC, mention should be made here of a new type of format which, by means of a head produced by thin-film technology, allows simultaneous area-like recording of up to 2000 tracks. This system is known as SDVCR. The potential of this system is particularly great in combination with an optical reading system, since small track widths of 2 xcexcm can be achieved and the storage density can be many times greater than in current systems.
The publication by Francouis Maurice xe2x80x9cTowards the Multitrack Digital Video Tape Recorderxe2x80x9d, J. Magn. Soc. Jpn, Vol. 15, Sup. No. 51 (1991), pages 389-394, discloses a process for recording digital signals on a magnetic tape in multiple longitudinal tracks by means of recording heads arranged in a matrix. The matrix comprises a ferrite block which is provided with grooves in the longitudinal and transverse directions and to which there is applied a thin-film structure, which contains the magnetically effective gaps. If current is passed through the windings let into the grooves, a magnetic field is created by the cumulative flux in all the ferrite legs. This flux finds its return primarily via the gap zone and magnetizes a magnetic tape located over the gap zone. The ferrite block combines many heads, which are manufactured together in one process.
By switched currents and correctly timed application of currents in the row and column directions, a magnetic field can be created in each individual gap. For this purpose, there is added to a signal applied to the data channels, which triggers winding currents in the rows, a rapidly switched selectioning current in the columns. The value of the magnetic flux resulting from the winding currents is chosen such that only the cumulative current from rows and columns can remanently magnetize the magnetic tape. Provided that the selectioning signal is chosen quickly enough, the stored energy in the inductance and the structure of the magnetized domains on the tape will not cause the drops in magnetization expected due to the intermittent losses of current to cause interference. The selectioning signal can consequently be used for various data lines in time-division multiplexing.
The tolerances occurring in the manufacture of recording heads in several, mutually separate production steps and in the assembly of this so-called matrix head leads to the individual recording heads being of very different quality with different magnetic properties of the recording heads of a matrix comprising ferrite block and applied thin-film gap, interference can occur due to stray fluxes.
The invention is based on the object of improving a method for the recording of digital signals on a magnetic tape in multiple longitudinal tracks by means of recording heads arranged in a matrix and a magnetic-tape recording apparatus for carrying out the process to the extent that different magnetic properties of the recording heads of a matrix are compensated.
The invention is based on the following considerations. With the currently used magnetic tapes of the ME (Metal Evaporated) and xe2x80x9cadvancedxe2x80x9d MP (Metal Particle) types, the reproduction amplitude initially increases as a function of the magnetic flux in the gap of the recording head with increasing magnetic flux, reaches a maximum and, falling slightly at first, goes over into a flat portion. The increase in crosstalk from one track to the neighbouring track as a function of the magnetic flux in the gap of the recording head in disproportionately great with increasing magnetic flux. An increase in the magnetic flux is consequently to a great extent uncritical for the reproduction amplitude, but leads to an increase in the magnetic crosstalk.
In practice, the optimum range of magnetic flux at which crosstalk is still tolerable is very narrow, with the result that the operating point of the recording head must be maintained very precisely. Since the magnetic flux in the gap of the recording head depends both on the magnetizing current and on the production-dependent magnetic properties of the recording heads, it is difficult to fix a magnetizing current at which all the recording heads have their operating points in the optimum range. Therefore, with previous solutions, proselected arrangements of recording heads having largely identical magnetic properties had to be used. This is where the solution according to the invention comes in, allowing recording heads which are arranged in a matrix and have varying magnetic properties of the individual recording heads, such as typically occur in mass production, to be used.
This takes place by determining for each individual recording head in preliminary tests an individual magnetizing current for which the operating point of the recording head lies in the optimum range. The values determined for these individual magnetizing currents are stored and, on selection of the recording heads, that is to say on activation by data signals, the selected recording heads are supplied with their individual magnetizing current. As a result, different magnetic properties of the recording heads of a matrix are electronically compensated.
In a magnetic-tape recording apparatus for carrying out the process, the storage of the values takes place by means of memories which are connected to a control input of a controllable current source assigned to the rows and columns. The current sources are preferably controllable constant-current sources.
The memories are preferably digital memories with a downstream analog/digital converter whenever the current sources have analog control inputs.
Suitable as memories are non-volatile memories, such as for example PROKs, battery-backed RMIs, ferromagnetic RAMs or EEPROMs.
According to a development, values determined in preliminary tests for compensation currents in neighbouring recording heads may be additionally stored and, on selection of one of the recording heads, the neighbouring recording heads are supplied with the stored values of the compensation currents. An a result, the magnetic crosstalk can be further reduced, taking into consideration different magnetic properties of the recording heads.
Three alternatives are possible for the storage of the magnetizing currents and also compensation currents. For instance, there may be stored exclusively the values of the row currents at constant column currents or exclusively the values of the column currents at constant row currents or both the values of the row currents and the values of the column currents. Although the first two alternatives reduce the outlay on memory modules, they do not always permit complete compensation. Their application is of use only for a small number of columns or rows. Preference is therefore given to the third alternative, which permits complete compensation even when there are a multiplicity of rows and columns.
The values of the row currents and/or column currents may be stored as absolute values or relative deviations from a basic value. In the case of the second variant, the basic value may be formed by a mean value or a lower limit value or an upper limit value.
While the storage of absolute values requires a high resolution and consequently a high storage capacity for each stored value, the storage of deviations and a basic value manages with a smaller storage capacity, in particular if deviations are small. In addition, the second variant allows a simple readjustment when there are changes to the magnetic properties of the recording heads caused by ageing or adaptations to other tape material, by changing just the basic value. A precondition here is that the magnetic properties of the recording heads do not change in relation to one another.
The basic values with the deviations may be added in digital form before the digital/analog conversion or in analog form after the digital/analog conversion.
In a preferred configuration of the magnetic-tap recording apparatus, the recording heads arranged in the matrix form with the controllable current sources, the memories and the control circuit a structural unit in which the recording heads are arranged on the side facing the magnetic tape and the current sources, memories and the control circuit are arranged on the side facing away from the magnetic tape. As a result, the subassemblies which serve for controlling the recording heads can be assigned directly to them and can switch them directly, with the result that both short current paths exist and a definite assignment of the memories to the columns and rows of the matrix is ensured on installation of the structural unit.